A conventional pneumatic gun is shown in FIGS. 1 to 4 and generally includes a barrel having an interior 14 and a piston unit 2 is movably received in the interior 14. A handle 1 is connected to the barrel and an inlet passage 10, a communication path 11, a release path 12, a chamber 13 and a free area 15 between the release path 12 an the chamber 13 are respectively defined. An inlet 16 is defined between the inlet passage 10 and the free area 15, and a neck opening 18 is defined between the chamber 13 and the free area 15. The communication path 11 and the interior 14 are in communication with each other so as to introduce air into the interior 14 to move the piston unit 2. A valve unit 3 includes a movable part 30, a piston member 31 and a seat 32 which is located at an end of the chamber 13. An end of the movable part 30 and the piston member 31 are located between the seat 32 and the chamber 13. An end of the piston member 31 protrudes out from the seat 32 and is located on the path of the movement of the trigger 4. The movable part 30 can be moved in the inlet 16, the free area 15 and the chamber 13. Two seals 35, 36 are connected on the movable part 30 and the movable part 30 is movable relative to the piston member 31.
When pulling the trigger 4, the piston member 31 is pushed upward as disclosed in FIG. 2 till the seal 37 seals the path of the movable part 30 to stop the air entering the chamber 13. The air in the chamber 13 releases from the gaps between the piston member 31 and the seat 32, and this forms a lower pressure area in the lower portion of the chamber 13 so that the movable member 30 is quickly moved downward till the seal 35 seals the neck opening 18 as shown in FIG. 3. The air is then allowed to enter the interior 14 via the inlet 16 and the communication path 11 to move the piston unit 2 to shoot a nail or a staple which is not shown.
It is noted that when shooting, the top end of the movable member 30 is located below the lowest periphery of the inlet 16 so that the inlet 16 is completely opened so that the air can enter the interior at a large volume to move the piston unit 2 at high speed. Nevertheless, the piston unit 2 moves so fast that the impact needle, the cushion pad, the piston and other parts of the piston unit 2 are easily damaged.
The present invention intends to provide a valve assembly for controlling the volume of air entering the interior of the barrel so as to protect the parts of the piston unit when shooting.